A vehicle (100) comprises a docking port and a selectively demountable switch (10) having a body and a rotary knob (40) on the body, and an interface between the switch and the docking port to enable commands entered by operation of the rotary knob to be communicated to the vehicle (100). The vehicle (100) incorporates a tow-assist system to facilitate reversing of a trailer, the vehicle (100) being of the type having a positive power-assisted steering mechanism (350) for its steerable wheels (250). The system comprises computing means (310) and the demountable switch (10). The computing means (350) is arranged to interpret rotational movement of the knob (40) on the body as desired steering movements of the trailer when the vehicle (100) is being reversed and to effect steering commands for the positive power assisted steering mechanism (350) to achieve said desired steering movements of the trailer.